The Guardrail airborne platform consists of a special electronic mission (SEMA) aircraft and its signals intelligence (SIGINT) payload. The SEMA aircraft are derivations from the U-21 Ute and C-12 Huron military utility aircraft. The airframe includes navigation, avionics and survivability equipment. A typical Guardrail mission requires the aircraft to orbit to the rear and parallel to the forward line of own troops (FLOT). The IPF sends commands to and receives information from the Airborne Relay Facility (ARF) through a secure data link. The operators in the IPF process the collected information and report the intelligence to the tactical commanders and other possible joint consumers via the JTT relay on board the aircraft.
In the 1971 time frame, the National Security Agency (NSA) desired to demonstrate the value of an airborne remote communications intelligence (COMINT) system. Doing a quick reaction capability (QRC) "stand-off Guardrail airborne signal collection/surveillance system" was a natural extension of the remote COMINT ground based systems that were used in the Vietnam war. A paper study and laboratory fly-off test showed that proven pre-selection digital receiver designs used in Vietnam along with the Explorer COMINT remoting would operate most effectively in the dense European signal environment. NSA and what was then Electromagnetic Sensor Laboratory, Inc (ESL) believed that these technologies could be deployed in an airborne system in less than 5 months, just in time for the '71 Return of Forces to Germany (REFORGER) exercises.
Between 1971 and 1972, 2 Guardrail Systems, and subvariant, were developed for use in Europe. Although the first 2 Guardrails, Guardrail I and II, were prototype systems, they met all their critical operational objectives. Formal documentation had to catch up later. However, the European Theater now had an airborne COMINT system that demonstrated the force multiplying factor that was to become part of the US/NATO defense strategy. A permanent Guardrail in Europe could provide peacetime data on a daily basis. Such a system could efficiently monitor military build-ups, track crisis development, be used to discover new threats, and be operationally ready for any hostilities that might come along.
In 1973, NSA initiated a program to develop a Guardrail capability for the Pacific Theater, which resulted in Guardrail IV. The Guardrail IV system was designed, built and tested on schedule, and on budget. The formal Technical Manuals for both Guardrail IV and Guardrail II were prepared and delivered as part of the Hardman Program along with additional spares in 1974. The Army Security Agency took the responsibility for supporting the fielded systems and maintained a small contingent of contractor Field Service Representatives. The factory was to be the depot for mission peculiar equipment, while SAAD and other military depots supported military standard items.
Each of the early Guardrail systems achieved its programmatic and operational requirements as to schedule, budget and operational performance. The early Guardrail systems were procured by NSA as QRC programs and were designated as theater level assets, which led to a long term requirement for Guardrail as an Army Corps level asset. Guardrail I through IV were considered to be theater level assets. This led to the development of the Guardrail V system in the late 1970s, its further development into the Improved Guardrail V system in the late 1980s, and finally the Guardrail / Common Sensor program in the early 1990s. By 1990, Guardrail were providing collection coverage along the inter-German border (starting in 1972), in Korea (starting in 1974), and in Central America (starting in 1983).